Proteins and Peptides Overview

Proteins

• Proteins are organic nitrogenous substances made up of carbon, hydrogen, oxygen, and nitrogen.
• The fundamental building blocks of proteins are amino acids.

Peptides

• A peptide is a chain of two or more amino acid residues linked by peptide bonds.
• A dipeptide is formed when two amino acids are joined together.
• Tripeptides are formed when three amino acids are linked.
• Polypeptides consist of 10 to 100 amino acid residues.
• Proteins are polypeptides with more than 100 amino acid residues.
• Aspartame, a dipeptide formed by aspartate and phenylalanine, is used as a sweetener.
• Glutathione, a tripeptide constituted by glutamate, cysteine, and glycine, plays a key role in detoxification.

Peptide Chain Directionality

• The peptide chain is unbranched and directional, starting with an amino group (N-terminus) on the left and ending with a carboxyl group (C-terminus) on the right.

Bonds Responsible for Protein Structure

• Covalent Bonds:
  • Peptide bonds
  • Disulfide bonds
• Non-Covalent Bonds:
  • Hydrogen bonds
  • Hydrophobic interactions
  • Electrostatic interactions
  • Van der Waals interactions

Peptide Bond

• Formed through the condensation of two amino acids.
• The peptide bond is formed between the alpha amino group of one amino acid and the alpha carboxylic group of another amino acid, resulting in an amide bond.

Characteristics of the Peptide Bond

• Partial double bond character
• Rigid structure
• Short length
• Planar shape (no free rotation)
• Polar nature
• Uncharged
• Present in the trans form

Disulfide Bonds

• Occur between two cysteine residues in either the same polypeptide chain or different chains.

Non-Covalent Bonds

• Weak bonds easily separated.

Hydrogen Bond

• Attractive interaction between polar molecules where hydrogen (H) is bound to a highly electronegative atom, such as nitrogen (N) or oxygen (O).

Hydrophobic Interactions

• Nonpolar side chains of neutral amino acids tend to cluster inside the protein molecule (away from water).
• Polar groups, on the other hand, are typically located on the exterior of the protein molecule (exposed to water).

Protein Structures

• Primary Structure: The number, type, and sequence of amino acids in a polypeptide chain, linked by peptide bonds. Primary structure is not affected by denaturation.
• Secondary Structure: Folding or bending of the polypeptide chain into specific structures. It is maintained by interactions between amino acids close to each other through hydrogen bonds. The most common secondary structures are the alpha helix and beta-pleated sheet.
• Tertiary Structure: The three-dimensional structure of a protein (protein folding). Tertiary structure is formed by a combination of disulfide bonds, hydrogen bonds, hydrophobic interactions, and electrostatic interactions. The two main forms of tertiary structure are fibrous and globular proteins.
• Quaternary Structure: The arrangement of multiple polypeptide chains (subunits) joined together by non-covalent bonds. An example is hemoglobin, which has four subunits.

Denaturation of Proteins

• Loss of the quaternary, tertiary, or secondary structure of a protein while the primary structure remains intact.
• Can be reversible or irreversible.
• Causes of denaturation include physical agents (e.g., excessive heating or radiation) and chemical agents (e.g., strong acids or alkalis).

Effects of Denaturation

• Decreased solubility
• Increased viscosity
• Loss of biological activity
• Changed antigenic properties
• Easier digestion

Classification of Proteins

• Shape:
  • Globular proteins: e.g., albumins and globulins
  • Fibrous proteins: e.g., keratin, myosin
• Biological Value:
  • High biological value: Complete set of essential amino acids.
  • Low biological value: Deficient in one or more essential amino acids, e.g., gelatin and plant proteins.
  • Structure:
    • Simple Proteins: Formed only from amino acids e.g. albumin.
    • Conjugated Proteins (Compound or Complex Proteins): Contain additional chemical components along with amino acids, e.g., hemoglobin.
    • Derived Proteins e.g., gelatin.

Explore the fundamental concepts of proteins and peptides, including their structure, amino acid composition, and significance in biological functions. This quiz covers peptide chains, their directionality, and the types of bonds that contribute to protein structure.